20 
ILLUSTRATED NATURAL HISTORY. 
CRABS : DIVISION AETICULATA. 
Every segment is supposed to be capable of bearing two pairs of appendages or members — one 
connected with the central, the other with the dorsal, portion of the segment. Both pairs of 
members do in fact occur upon all or a portion of the segments, in some of these animals ; but in 
general, the ventral members alone are developed, and these only on certain segments. In the 
insects, in addition to three pairs of ventral members or legs, we find generally two pairs of dorsal 
appendages — the wings. Sometimes, as in the earthworm and leech, the limbs are entirely defi- 
cient, or only represented by a few bristles ; but, when present, their number is never less than six. 
The nervous system of the Articulata generally exhibits the tendency to segmentary repetition, 
characteristic of the group, very distinctly. In its most characteristic form, it consists of a double 
nervous cord running down the middle of the ventral portion of the body, and uniting a series of 
knots or ganglia which lie in its course : these ganglia give rise to nerves which are distributed 
to the various organs. The more elongated the body, and the more similar the different seg- 
ments of which it is composed, the more regularly do the ganglia follow one another ; while, 
when the segments become more or less amalgamated, the individual ganglia fuse in a correspond- 
ing degree into larger masses. This ventral cord originates from one or more cephalic gangha 
of considerable size, situated in the head above the oesophagus, which give off two filaments to 
join the first ventral ganglion, and thus form a nervous ring surrounding the oesophagus. From 
this the ventral cord takes its rise. 
In the lowest animals arranged in this division we have some difiiculty in referring the nervous 
system to the articulate type ; but when these animals present us with a distinct nervous system, 
it consists of one or two ganglia situated in the neighborhood of the oesophagus, and giving off 
two thin branches which run down the body. 
The majority of the Articulata possess the senses in tolerable perfection. The eyes in many 
cases present a highly complex structure, consisting of a great number of hexagonal facets, each of 
which may be regarded as a distinct eye ; this construction of the eyes is especially prevalent in 
insects, and is peculiar to the annulose division. When these eyes are wanting, and even when 
they are present, we frequently meet with simple eyes, which agree very closely in structure with 
the individual eyes, by the aggregation of which the compound visual organs are formed. The 
senses of hearing, taste, and smell appear also to be possessed by a great many of these creatures ; 
but the organs by which these faculties are exercised can seldom be indicated with any degree of 
